docs: def elliconstraint(self, x, cfac=1e8, tough=True, cond=1e6):
          """ellipsoid test objective function with "constraints" """
          N = len(x)
          f = sum(cond**(np.arange(N)[-1::-1] / (N - 1)) * x**2)
          cvals = (x[0] + 1,
                   x[0] + 1 + 100 * x[1],
                   x[0] + 1 - 100 * x[1])
          if tough:
              f += cfac * sum(max(0, c) for c in cvals)
          else:
              f += cfac * sum(max(0, c + 1e-3)**2 for c in cvals)
          return f